The Environmental Protection Agency has instituted stringent automobile-related environmental regulations. A primary focus of the regulation relates to the Corporate Average Fuel Economy (CAFE) standards, which mandate a specified, gradual increase of a corporate fleet's overall fuel economy by the established target dates. CAFE standards have spurred industry wide research and development of "lean-burn engines". The term "lean-burn engine" used herein is defined as an engine utilizing air/fuel mixtures having an oxygen content in excess of the stoichiometric air/fuel ratio (lean mixtures). The use of such lean air/fuel mixtures, reduces the consumption of fuel and thus enhances an automobile's fuel economy. The effort to increase automobile fuel economy has become paramount in the wake of the Environmental Protection Agency's tightening CAFE standards.
In addition to the CAFE standards, the Environmental Protection Agency has set a schedule for the continual reduction of specific automotive emissions. Thus, the Environmental Protection Agency's regulatory measures have required the automobile industry to simultaneously increase fuel economy while decreasing harmful exhaust emissions. Accordingly, there exists a new found interest in the development of a catalyst to operate efficiently under lean-burn conditions.
Many of the prior art catalysts were designed to optimally operate at or about stoichiometric conditions. These prior art catalysts when combined with a lean fuel mixture result in O.sub.2 being adsorbed by the catalyst, preventing NO.sub.x from being reduced to nitrogen (N.sub.2) by the active metal on the catalyst. So while the current three-way catalyst, for example, may effectively reduce NO, hydrocarbons (HC) and CO emissions at stoichiometric conditions, the efficiencies of a three-way catalyst for NO.sub.x reduction diminish significantly in an oxygen rich environment.
The effectiveness of a catalyst is largely dependent on the air/fuel ratio of the fuel mixture which is employed in an engine. If a lean mixture is utilized, a catalyst exhibits a high activity of oxidation but a low activity of reduction, while the combustion product contains a large amount of oxygen. In contrast, in the presence of a fuel rich mixture, a catalyst exhibits a high activity of reduction but a low activity of oxidation. At stoichiometric air/fuel ratios, oxidation and reduction are balanced.
Accordingly, there is a need for a lean-NO.sub.x catalyst which efficiently reduces NO.sub.x emissions in the presence of a lean fuel mixture. In addition to automotive applications, lean-NO.sub.x catalysts are also valuable in lowering the emissions of stationary power plants that burn fossil fuels. A lean-NO.sub.x catalyst is defined for purposes of this application as a catalyst that can reduce NO.sub.x under lean-burn conditions.
In answer to this problem, transition metal-containing zeolite catalysts have been developed to reduce NO.sub.x emissions for lean mixtures. In particular, copper-zeolite catalysts have been preferred due to the effectiveness of copper as an ion exchange metal at lower temperatures such as that present in lean-burn engine exhaust.
Zeolites are crystalline aluminosilicates. Zeolites are commercially available and are characterized by crystal structures having different channels or pore diameters. Zeolites have fine pores (sized at several angstroms), comparable with the size of a molecule, hence they are termed molecular sieves. It is believed that the hydrocarbons are selectively trapped in those pores or sieves. As the transition metal, introduced by ion exchange, forms active sites in the pores, the hydrocarbons are adsorbed therein and react with nitrogen oxides in the presence of excess O.sub.2. Accordingly, zeolite catalysts have potential application in the removal of NO.sub.x from lean fuel mixtures.
The basic principle behind removing NO.sub.x in a lean fuel mixture with a transition metal, e.g., copper, copper-containing zeolite catalyst relies on adsorbing NO.sub.x and effectuating a catalytic reaction of the adsorbed NO.sub.x with the hydrocarbons contained in the exhausted gas, to reduce the NO.sub.x to N.sub.2. Although copper-zeolite catalysts have good catalytic activity initially, due to copper's high NO.sub.x adsorption ability, copper-zeolite catalysts have a series of limitations which are described below.
First, transition metal containing zeolites degrade at high temperatures usually found in automotive exhaust systems. Furthermore, if the zeolite catalyst is exposed to steam-containing air, e.g., automotive exhaust, the activity of the catalyst decreases rapidly. The decrease in activity is also accompanied by dealumination. Steam deactivation of zeolite-based catalysts is the primary reason why such catalysts are not practical in application.
To compensate for the problems associated with zeolites, alumina (Al.sub.2 O.sub.3) has been used in three-way catalysts as support for active ingredients. The use of Al.sub.2 O.sub.3 and metal containing Al.sub.2 O.sub.3 as lean-NO.sub.x catalysts has been reported by H. Hamada, Y. Kintaichi, M. Sasaki and T. Ito in "Transition metal-promoted silica and alumina catalysts for the selective reduction of nitrogen monoxide with propane", Applied Catalysts, L1-L8 (1991). Alumina, however, is inefficient in NO.sub.x reduction such that it is currently impractical for commercial use.
Thus, a need exists for a commercially practical, efficient, lean-NO.sub.x catalyst as provided by the present invention.